Flat panel displays have been used for various products in various fields in recent years, and are required to have a larger size, a higher definition, and a less power consumption.
In view of the circumstances, organic EL display devices including organic EL elements utilizing electro luminescence (hereinafter, referred to as “EL”) of an organic material draw significant attention as all-solid flat panel displays having excellent low-voltage driving, excellent high-speed response, excellent self-emitting, and an excellent wide-viewing-angle property.
For example, an organic EL display device is configured such that an organic EL element which is electrically connected to a thin film transistor (TFT) is provided on a substrate made from a glass substrate or the like on which the thin film transistor is provided.
The organic EL element is a light emitting element capable of emitting light having a high luminance by low-voltage direct-current driving, and includes a first electrode, an organic EL layer, and a second electrode which are stacked in this order.
As a method for achieving full-color display in an organic EL display device including such an organic EL element, there are known, for example, a first method involving arraying, as sub-pixels, organic EL elements, which emit red (R) light, green (G) light, and blue (B) light, on a substrate and a second method involving selecting a color of light for each sub-pixel by incorporating an organic EL element emitting white light with a color filter.
In those methods, there has been proposed in recent years a method for improving a chromaticity of emitted light and a light-emitting efficiency by utilizing a microcavity effect (e.g., see Patent Literatures 1, 2).
The microcavity is a phenomenon in which, by multiply reflecting emitted light between an anode and a cathode to resonate the emitted light, an emission spectrum is steeply changed, and a light intensity of a peak wavelength is amplified.
The microcavity effect can be obtained by, for example, optimally designing reflectivities and film thicknesses of anode and cathode, a layer thickness of an organic layer, etc.
As a method for introducing such a resonance structure, i.e., a microcavity structure into an organic EL element, for example, there is known a method involving changing, for each emission light color, optical path lengths of organic EL elements in respective sub-pixels.
As the method for changing, for each emission light color, the optical path lengths of the organic EL elements in the respective sub-pixels, there is a method involving stacking an organic EL layer including a light-emitting layer and a transparent electrode layer between a reflecting electrode and a semitransparent electrode.
That is, for example, in a case of a top emission organic EL element, there is a method involving (i) forming an anode by stacking a reflecting electrode layer and a transparent electrode layer and (ii) changing, for each sub-pixel, a film thickness of the transparent electrode layer provided on the reflecting electrode layer of the anode.
In a case of the top emission organic EL element, the microcavity structure can be introduced by (i) forming the anode by stacking the reflecting electrode layer and the transparent electrode layer as described above, (ii) stacking an organic EL layer appropriately, and (iii) forming a cathode with a semitransparent electrode made from, for example, a semitransparent silver which has been shaped into a thin film.
In a case where the microcavity structure is introduced into the organic EL element as described above, a spectrum of light emitted from the light-emitting layer through the cathode is more steeply changed as compared with a case where the organic EL element does not have the microcavity structure, and an emission intensity of the light to a front surface is largely increased.
Each of Patent Literatures 1 and 2 discloses an organic EL display device in which the microcavity structure is introduced into an organic EL element by stacking transparent electrode layers which are made from the identical material but whose number is different between sub-pixels.